1. Field of the Invention
This invention relates to a semiconductor laser having a doubleheterostructure on a striped channel substrate.
2. Description of the Prior Art
(Semiconductor lasers for laser oscillation at a long wavelength of visible light)
In the preparation of the inner stripe geometric structure of a semiconductor laser for laser oscillation at a long wavelength of 750 nm or more or of other semiconductor devices, a crystalline layer is grown on a V- or U-shaped channel substrate by an epitaxial growth technique, wherein the channel in the substrate must not be deformed during the epitaxial growth of the layer. When molecular beam epitaxy (MBE) or metal organic-vapor phase epitaxy (MO-VPE) is used as the epitaxial growth technique, the original shape of the channel is maintained. However, since the crystalline layer is formed in accordance with the shape of the channel in the substrate, the surface of the resulting growth layer cannot be a plane.
On the other hand, if a substrate 1 which is etched, in advance, to form the V-shaped channel as is shown in FIG. 1 is provided, and a crystalline layer 3 is grown on this substrate using liquid phase epitaxy (LPE) as the epitaxial growth technique to completely fill the channel 2, the formation shown in FIG. 2 is created, resulting in a growth layer having a plane surface. However, each of the shoulder portions at both sides of the channel 2 undergoes meltback resulting in a rounded shape, so that the width W (shown in FIG. 1) of the channel 2 expands to W' (shown in FIG. 2). This is because the growth rate of the crystalline layer growing initially in the bottom of the channel 2 is so high that there is an insufficient amount of solute in the shoulder portions of the channel 2, so that meltback of the shoulder portions of the channel 2 occurs. The expansion of the width of the channel 2 from W to W' is undesirable. Moreover, impurities from the substrate 1 are mixed into the growth layer 3 resulting in inferior characteristics of the device.
(Semiconductor lasers for laser oscillation at a short wave length of visible light)
In the preparation of the inner stripe geometric structure of a semicondcutor laser for laser oscillation at a short wavelength, the first cladding layer is used to fill the channel in the substrate, resulting in the following drawbacks:
FIG. 3 shows a VSIS (V-shaped channel substrate inner stripe) semiconductor laser with a plane active layer, which was disclosed in TGED 81-42, 31 (1981-July) IECE Japan, comprising a p-GaAs substrate 11, an n-GaAs current blocking layer 12, a p-Ga.sub.1-y Al.sub.y As cladding layer 13, a p-Ga.sub.1-x Al.sub.x As active layer 14, an n-Ga.sub.1-y Al.sub.y As cladding layer 15 and an n-GaAs cap layer 16. The V-shaped channel 17 serves as an electroconductive region. In order that this VSIS laser having an inner stripe geometric structure attains laser oscillation at a wavelength of 750 nm or less, the mole ratio y of AlAs to GaAs in the cladding layers 13 and 15 must be 0.6 or more. When the mole ratio is less than 0.6, carriers leak out of the active layer 14 to the cladding layers 13 and 15 resulting in an increase in the threshold current and a decrease in the dependency of the threshold current upon temperature. Especially, extensive leakage of carriers (i.e., electrons) from the active layer 14 to the p-cladding layer 13 tends to take place, so that a considerably high barrier must be set to prevent such leakage of electrons to the p-cladding layer 13. The height of the barrier depends not only upon the mole-ratio y of the AlAs to GaAs by also the positive hole concentration of the p-cladding layer 13. For example, when the positive hole concentration is 5.times.10.sup.17 cm.sup.-3, the barrier must be raised to aproximately 50 meV (i.e., a 20 percent raise of the original barrier). This means that the positive hole concentration of the p-cladding layer 13 must be at least 1.times.10.sup.18 cm.sup.-3.
In order to obtain the carrier (positive hole) concentration at 1.times.10.sup.18 cm.sup.-3 or more in the p-Ga.sub.1-y Al.sub.y As cladding layer 13 wherein y is greater than 0.6, Mg is preferably used as a p-impurity since the addition of Mg in a concentration of 0.3 atomic percent or more to the p-cladding layer attains a carrier (positive hole) concentration of 1.times.10.sup.18 cm.sup.-3 or more. However, when Mg is added to the Ga.sub.1-y Al.sub.y As to a great extent, the growth rate of the crystalline layer decreases because Mg functions to reduce the diffusion coefficient of As in the Ga solution. Especially, when Mg in a concentration of 0.3 atomic percent is added to Ga.sub.1-y Al.sub.y As under the condition of y&gt;0.6, the growth rate of the Ga.sub.1-y Al.sub.y As slow significantly, so that, as shown in FIG. 4, the channel 17 will be insufficiently filled with the p-cladding layer 13, resulting in an active layer 14 having a concave surface which induces the high-order transverse mode. Even though the growth period of the p-cladding layer 13 is extended during the growth process, the formation of an active layer 14 having a plane surface is quite difficult. Even if the plane surface of the active layer 14 results from the extension of the growth period, the portion of the p-cladding layer 13 on the outside of the channel 17 becomes too thick to serve as a waveguide. As mentioned above, the addition of Mg in the selected concentration to the p-cladding Ga.sub.1-y Al.sub.y As (y&gt;0.6) layer in a conventional manner to obtain a positive hole concentration of at least 1.times.10.sup.18 cm.sup.-3 in the p-cladding layer, cannot attain a good yield of semiconductor lasers having excellent characteristics and good reproducibility of the characteristics of the lasers.